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Cisco Bug: CSCvv85908 - BGP to recover gracefully after label collisions due to "local-as" misconfiguration

Last Modified

Sep 29, 2020

Products (1)

  • Cisco ASR 9000 Series Aggregation Services Routers

Known Affected Releases

6.4.2.ROUT

Description (partial)

Symptom:
Label Collision messages seen on BGP. Traffic forwarding to those prefixes impacted.
RP/0/RSP0/CPU0: bgp[1067]: %ROUTING-BGP-4-LABEL_COLLISION : Label 24001 collision: prev: [T: 3 RD:0:0:0 PFX/NHID:10.205.2.230/32] curr: [T: 13 RD:0:0:0 PFX/NHID:10.205.2.230/32]

MPLS forwarding information looks to not be in synch with LDP:

RP/0/RSP0/CPU0:F340.07.21-ASR-9001-1#show mpls forwarding labels 24001 detail

Local  Outgoing    Prefix             Outgoing     Next Hop        Bytes
Label  Label       or ID              Interface                    Switched
------ ----------- ------------------ ------------ --------------- ------------
24001  Pop         10.205.2.230/32    Te0/0/2/0    10.205.24.81    0


RP/0/RSP0/CPU0:F340.07.21-ASR-9001-1#show mpls ldp ipv4 forwarding local-label 24001

Codes:
  - = GR label recovering, (!) = LFA FRR pure backup path
  {} = Label stack with multi-line output for a routing path
  G = GR, S = Stale, R = Remote LFA FRR backup

Prefix          Label   Label(s)       Outgoing     Next Hop            Flags
                In      Out            Interface                        G S R
--------------- ------- -------------- ------------ ------------------- -----
10.205.2.230/32 24001   16001          Te0/0/2/0    10.205.24.81

Conditions:
1. IOS-XR DUT (device under test) is being used as an ASBR for inter-AS connection.
2. Both AS have the same AS number.
3. IOS-XR DUT requires the local-as command to peer using iBGP with local-AS neighbors.
4. This defect is hit if local-as command is missing to peer with local-AS iBGP neighbors. From IOS-XR DUT perspective, local-AS neighbor will peer using eBGP and consider it as an Option-B ASBR router leading to label_collision messages and potential packet loss.
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